Crude kaolin clay, as mined, contains various forms of discoloring elements, two major impurities being anatase (TiO.sub.2) and iron oxides. In order to make the clay more acceptable for use in the paper industry, these impurities must be substantially removed by appropriate techniques. The production of high brightness clay usually includes two processing steps. In the first step, a significant portion of the impurities, mainly anatase, is removed by employing one or two physical separation techniques, such as high gradient magnetic separation (HGMS), froth flotation and selective flocculation. In the second step, the remaining impurities, mainly iron oxides, are removed by chemical leaching.
Froth flotation is regarded as one of the most efficient methods of removing colored impurities from clay, although some variations may be necessary for improved results. For example, the use of carrier particles or oil droplets to improve fine particle flotation has been suggested in U.S. Pat. Nos. 2,990,958 and 3,432,030, respectively. Nevertheless, practically all of the known flotation processes are based on the use of the fatty acidor tall oil-type of reagents called "collectors" that are designed to render the colored impurities selectively hydrophobic. Use of these reagents, however, requires the use of monovalent, divalent, or trivalent cations called "activators". This makes the process sometimes difficult to control as it is necessary to maintain a proper balance between the amounts of collector and activator added. An excessive use of activators can induce coagulation of the clay particles and makes the separation difficult. Also, activators can cause the flotation of the clay particles themselves rather than the colored impurities, resulting in a poor separation efficiency and a loss of clay recovery.
It is therefore, desirable to have a collector for colored impurities that does not require activators. It has been reported (Marabini and Rinelli, AIME Preprint No. 82-50, February, 1982) that N-phenylbenzohydroxamic acid can be used as a collector for rutile, a polymorph of anatase, without the use of activators. The flotation of rutile using this reagent occurs at acidic pH values, however, and substantially no flotation is possible above pH 5. This result makes it difficult to remove impurities from clay because in acidic media, clay particles self-coagulate to form cages in which impurities are trapped. For this reason, physical separation processes involving kaolin clay are carried out in an alkaline, or only slightly acidic, medium in which the clay particles can be more readily dispersed. In addition, N-phenylbenzohydroxamic acid is prohibitively expensive and exceedingly large amounts of the reagent are required for good flotation.
U.S. Pat. No. 3,438,494 discloses the use of alkyl- or aryl-substituted hydroxamic acids or salts thereof as collectors for the flotation of chrysocolla, a copper-bearing silicate mineral, and iron oxides from ores containing these minerals. Similarly, potassium octyl hydroxamate has been reported (LeNormand, Salman and Yoon, Can.Met.Quarterly, Vol. 18, pp. 125-129) to be useful as a collector for the flotation of malachite, an oxidized copper mineral. No activators are necessary for the flotation of these minerals using hydroxamates, since these reagents are chelating agents specific for copper and iron. Neither of these references, however, suggests the use of hydroxamates for the flotation of titaniferous impurities from kaolin clays.